Compressed air energy storage (CAES) systems have been used for storing energy in the form of compressed air, for example, in electric energy time shift applications, wherein air is compressed and stored when electrical demand is low, and expanded when electrical demand is high, to recover the stored energy and convert it to electricity. CAES systems typically comprise an air compressor, with one or multiple stages and cooling after each stage, large-volume storage such as an underground cavern, and an expander, with one or multiple stages and heating prior to each expansion stage. The expanded air in CAES can also be mixed with fuel and fed to a combustion turbine to increase its efficiency. On the latter types of systems, the fuel (either natural gas, or other combustible gases), depending on the supply pressure, may require compression in order to reach the turbine admission pressure.
Compressed gases, such as carbon dioxide, nitrogen, oxygen, hydrogen, helium, air, and others, have been used for various industrial and other applications. One of the methods available to transport these gases to their end users involves storing them in high pressure containers to considerably reduce the gas volume. This is usually achieved by using a compression system to fill the gas containers to a desired pressure. The gas is typically treated before each compression stage to remove possible liquids, and cooled after each compression stage to avoid exceeding the maximum temperature allowed by the compression system, and also to maximize the mass of gas inside the container. The filling stage may also require temperature compensation, depending on the environmental conditions, to ensure that pressure variations inside the container resulting from changes in environmental temperature are maintained within allowable ranges. At the user end, the compressed gas inside the containers is typically expanded using a pressure regulator to the desired utility pressure.
One particular form of compressed gas that is increasingly used in transportation and power generation applications is compressed natural gas (CNG). CNG has been used as a source of fuel for natural gas vehicles (NGVs), thermal power generation plants, as well as for transportation of natural gas between different locations where a natural gas pipeline is not feasible (so-called “virtual pipeline” systems). NGV fueling stations typically involve multiple processes including gas odorization, treatment to remove solids, condensates and water, compression (in one, or various stages, including aftercooling and liquids removal), and finally, utilizing a dispenser system to fill the CNG bottles inside the NGVs. Further these processes may also include buffer storage depending on the arrangement of the station (fast-fill, or time-fill). Virtual pipeline systems typically include a compression, or mother station, where CNG bottles are filled, and decompression, unloading, or daughter stations, where the CNG bottles are discharged for end use.
On a larger scale, storage of natural gas at high pressures, typically in underground caverns, has also been used as a means to store natural gas, mostly to balance periods of high and low demand for the fuel. However, these types of systems are not applicable to many locations, as they require specific geological conditions.
The systems and methods described for CAES applications typically only use compressed air as the form in which energy is stored for later conversion to electrical energy. They do not use other available industrial or fuel gases. Furthermore, these systems are generally conceived to compress, store and expand the air at the same location, and do not have the flexibility to permit the compression and expansion stages to be placed at different locations. The systems and methods described for containerized compressed gases have been conceived and used for the storage and transportation of different industrial or fuel gases for their final use. However, they have not been conceived to convert and later produce electrical energy in a manner similar to how CAES stores the energy in compressed air. Further, the systems and methods for large scale storage require specific geologic at the site, and are not applicable where there is a lack of appropriate geologic formations such as underground caverns.